Type 3 secretion effectors - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 453

bacterial attachment site ( Miyahara et al., 2009 ). EspL2 also seems to have a
Tir-independent function whereby it induces pseudopod-like protrusions of the
host cell membrane to enhance bacterial adherence ( Miyahara et al., 2009 ). EspL
proteins are homologs of the Shigella OspD family, however the Shigella OspD
proteins do not appear to be involved in actin aggregation ( Nataro et al., 1995 ).
Like its distant homolog TccP/EspFu, EspF possesses N-WASP binding
sites, which have been shown in vitro to be functional ( Alto et al., 2007 ). EspF
also binds to sorting nexin 9 (SNX9) and induces membrane remodeling and
in vitro can nucleate a multiprotein complex of both N-WASP and SNX9 to
potentially coordinate membrane remodeling and actin polymerization ( Alto
et al., 2007 ).
Invasion and intracellular spread
The uptake of Shigella by the host cell depends on a complex rearrangement of
the host cell membrane and cytoskeleton, which is initially triggered by recep-
tor binding but requires the T3SS effectors for complete uptake ( Figure 15.1 B).
Recently it has been shown that bacteria can also be captured by filopodial
extensions from the host cell, termed nanometer-thin micropodial extensions
(NMEs), and this interaction occurs through the T3SS tip complex proteins
IpaB and IpaD ( Romero et al., 2011 ). Connexin-mediated signaling and extra-
cellular ATP stimulates Erk1/2 activation, which controls actin retrograde flow
in NMEs resulting in NME retraction, bringing the bacterium into contact with
the cell body to allow invasion to occur.
Bacterial invasion begins with the formation of membrane ruffles, which
require modulation of Rho GTPase activity by IpgB1 and 2. Rho GTPase
modulation is discussed in detail elsewhere in this chapter. In addition, the
carboxylterminal domain of IpaC induces actin polymerization responsible for
the formation of cell extensions that engulf the bacterium ( Tran Van Nhieu
et al., 1999 ; Kueltzo et al., 2003 ). IpaC appears to do so by recruiting and
activating Src tyrosine kinase ( Mounier et al., 2009 ). IpaC also forms part of
the T3SS pore and is thus necessary for delivery of T3SS effector proteins into
the host cell. A Shigella strain mutated in the IpaC C-terminal domain was
proficient in translocating T3SS effector proteins but was unable to recruit Src,
uncoupling these two functions of IpaC ( Mounier et al., 2009 ). Src appears to
be initially recruited to the intimate contact site of the bacteria but quickly dif-
fuses in an actin-dependent manner into extensions that surround the bacteria
( Dumenil et al., 1998 ), the molecular details governing this process remain to
be elucidated.
The T3SS effector IpgD dephosphorylates phosphatidylinositol-4,5-biphos-
phate [PI(4,5)P(2)] into phosphatidylinositol-5-phosphate [PI(5)P] ( Niebuhr
et al., 2002 ). This dephosphorylation event leads to a decrease in membrane
tether force, which can be seen by increased membrane blebbing and actin fila-
ment remodeling upon ectopic expression of the protein ( Niebuhr et al., 2002 ).